The present invention relates to the measurement of the progress of a chemical reaction that generates an electroactive reaction product that is subsequently detected at an electrode amperometrically or coulometrically. The method is useful in applications where it is desirable to follow the progress of a chemical reaction, particularly in sensor applications where the progress of the reaction of an analyte can be useful in determining the analyte concentration. Sensors used in such applications may include a working electrode (2) and a counter electrode (3) disposed on an electrically insulating substrate (1), and may also include a second substrate (5) upon which is disposed a layer of chemical reactants (4).